1. Field of the Invention
The present invention relates to mobile communications. More particularly, the present invention relates to a downlink transmission/reception apparatus and method for a mobile communication based on Orthogonal Frequency Division Multiple Access (OFDMA).
2. Description of the Related Art
Mobile communication systems have evolved beyond the early voice-oriented services and now include a high-speed, high-quality wireless packet data communication system to provide data and multimedia services. In this regard, various mobile communication standards, such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced (LTE-A), defined in the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) defined in the 3rd Generation Partnership Project-2 (3GPP2), and 802.16 defined by the Institute of Electrical and Electronics Engineers (IEEE), have been developed to support the high-speed, high-quality wireless packet data communication services. More particularly, LTE has been developed and is the most promising technology that is capable of facilitating the high speed packet data transmission and maximizing the throughput of the radio communication system with various radio access technologies. LTE-A is the evolved version of LTE to improve the data transmission capability.
LTE corresponds to the 3GPP release 8 and release 9 standards while LTE-A corresponds to the release 10 standard. The 3GPP continues to study further advancements for LTE-A and to release standards following LTE-A.
The existing 3rd generation wireless packet data communication systems, such as HSDPA, HSUPA and HRPD, use technologies such as Adaptive Modulation and Coding (AMC) and Channel-Sensitive Scheduling to improve the transmission efficiency. With the use of AMC, a transmitter can adjust an amount of data transmission according to the channel state. That is, when the channel state is ‘Bad’, the transmitter reduces the amount of data for transmission to match the reception error probability to a desired level, and when the channel state is ‘Good’, the transmitter increases the amount of data for transmission to transmit a large volume of information efficiently while matching the reception error probability to the desired level.
Using the channel-sensitive scheduling resource management method, the transmitter, since it selectively services a user having a superior channel state among several users, can increase the system capacity, as compared with a transmitter that allocates a channel to one user and services the user with the allocated channel. Such capacity increase is commonly referred to as a multi-user diversity gain. In brief, the AMC method and the channel-sensitive scheduling method are methods for receiving partial channel state information being fed back from a receiver, and applying an appropriate modulation and coding technique at the most efficient time that is determined depending on the received partial channel state information.
LTE and LTE-A have adopted Orthogonal Frequency Division Multiple Access (OFDMA) as the multiple channel access mechanism. The 3GPP and 3GPP2 have adopted OFDMA for the advanced systems. OFDMA is expected to provide superior system throughput as compared to Code Division Multiple Access (CDMA). One of the main factors that allows OFDMA to increase system throughput is the frequency domain scheduling capability. As channel sensitive scheduling increases the system capacity using the time-varying channel characteristic, OFDMA can be used to obtain more capacity gain using the frequency-varying channel characteristic.
In order to expand the total service coverage area, a cellular mobile communication system operates with a plurality of cells that provide terminals with a communication service within the service coverage area of each cell.
FIG. 1 is a diagram illustrating a cellular concept of a mobile communication system according to the related art.
Referring to FIG. 1, a cellular system including three cells is illustrated. Each cell is provided with a transceiver facility to provide a terminal (e.g., User Equipment (UE)) with mobile communication service within the service coverage area of the cell. Each of the transceiver facilities of the base stations (e.g., evolved Node Bs (eNBs)) 100, 110, and 120 have service coverage areas with a radius of a few hundred to a few thousand meters.
Such a cellular topology is advantageous to provide mobile communication service over a large area. At the initial system configuration stage, a plurality of base stations are deployed to secure a large service coverage area (i.e., service provision area) as shown in FIG. 1. As the use and amount of mobile data increases, the mobile communication system evolves to meet the user requirements. The system evolvement is achieved with the miniaturization of the cell size as well as the adoption of new transmission techniques and an increase in the number of antennas. For example, femto cells are deployed at hot spots accommodating a high volume of data traffic.
FIG. 2 is a diagram illustrating an architecture of a mobile communication system according to the related art.
Referring to FIG. 2, a cellular communication system having a plurality of femto cells deployed within the macro cells is illustrated.
In FIG. 2, a plurality of small cells 240 are deployed in the macro cells 230 formed by the base station transceiver facilities 200, 210, and 220. The femto cells 240 provide the terminals with mobile communication service within their service areas at low transmit power. A femto cell 240 and a macro cell 230 have the following differences:                The femto cell 240 performs downlink transmission at low transmit power while the macro cell 230 performs downlink transmission at higher transmit power.        The femto cell 240 is deployed to provide service to terminals moving in a small area at low mobility while the macro cell 230 is deployed to provide service to terminals moving in a relatively large area at high mobility.        
The difference in transmit power between the femto and macro cells influences the delay spread of a signal propagating over a radio channel to some extent. The delay spread corresponds to the time delay between the arrival time of a signal that has been reflected by various obstacles. The delay spread can be influenced by the signal's transmit power because the magnitude of the signal's reflectance corresponds to the amount of power used to transmit the signal. That is, the higher the transmit power, the farther the reflected signal. In contrast, a lower transmit power results in less reflectance and hence a shorter delay spread.
The difference in mobility supported between the femto and macro cell is required because the macro cell 230 has to provide a mobile communication service commonly to all of the terminals within the cell coverage regardless of the mobility category of the terminals. Typically, a femto cell 240 aims to provide a mobile communication service in a population density area such as a downtown area, a shopping mall, a sports complex, and the like, while the macro cell 230 aims to provide a mobile communication service within a relatively large service coverage area even to a terminal in a vehicle moving on the highway at high speed. The speed of the terminal determines whether the signal arriving at the terminal experiences a certain fading in time. In the case of a terminal moving at low speed, the delay spread can be assumed unchangeable in the transmit time interval as the unit of signal transmission of the base station. Oppositely, in the case of a terminal moving at high speed, the delay spread may change irregularly in the transmit time interval of the base station, resulting in fading.
Since the downlink signal received by the terminal experiences different radio channels according to whether it is received in the femto cell 240 or the macro cell 230, as a consequence, the received signal is distorted. The delay spread is relatively short in the femto cell 240 with regular fading in the transmit time duration while it is relatively long in the macro cell 230 with irregular fading in the transmit time interval.
In a common mobile communication system, the macro and femto cells 230 and 240 operate using the same mobile communication protocol. That is, the downlink transmission is performed in the same frame format in both the macro and femto cells 230 and 240. In the case of the macro cell 230, it is necessary to allocate a large amount of radio resources to overcome a relatively long delay spread and the time-varying fading in the transmit time interval. For example, OFDMA, which is used as the multiple access of LTE/LTE-A as the 4th generation mobile communication standard, uses 1/15 of the entire radio resource for suppressing the performance degradation caused by the delay spread. Also, in order to compensate for the time varying fading in the transmit time interval, ⅓ of the entire radio resource is used. The resource assignment for overcoming the delay spread or time varying fading in the transmit time interval results in the reduction of the amount of resources available for real data transmission. It degrades the resource utilization efficiency to perform the downlink transmission with the same frame format in the macro and femto cells 230 and 240 regardless of the different transmission environment of the macro and femto cells.